Myocardial cells in tissue culture will be studied by employing techniques of laser microbeam irradiation, electron microscopy and electrophysiology. Electron microscope studies will involve examination of the target-irradiated mitochondrion as well as other cell organelles and the cell surface. In addition, various ultrastructural cytochemical and autoradiographic studies will be conducted. The altered contractile states of the irradiated cell and associated myocardial cells will be investigated with respect to induction, course of the response, and recovery. Comparisons will be made between auricle and ventricle cells from hearts of different age rats (neonatal and embryonic) and other vertebrates. Studies also will be undertaken to elucidate the biophysical mechanisms of laser light interaction with biological systems.